400nm ultra-broadband gratings for near-single-cycle 100 Petawatt lasers

Compressing high-energy laser pulses to a single-cycle and realizing the “λ3 laser concept”, where λ is the wavelength of the laser, will break the current limitation of super-scale projects and contribute to the future 100-petawatt and even Exawatt lasers. Here, we have realized ultra-broadband gold gratings, core optics in the chirped pulse amplification, in the 750–1150 nm spectral range with a > 90% −1 order diffraction efficiency for near single-cycle pulse stretching and compression. The grating is also compatible with azimuthal angles from −15° to 15°, making it possible to design a three-dimensional compressor. In developing and manufacturing processes, a crucial grating profile with large base width and sharp ridge is carefully optimized and controlled to dramatically broaden the high diffraction efficiency bandwidth from the current 100–200 nm to over 400 nm. This work has removed a key obstacle to achieving the near single-cycle 100-PW lasers in the future.

The manuscript "400 nm ultra-broadband gratings for near-single-cycle 100 Petawatt lasers" by Yuxing Han et al. provides a new grating design intended for scaling peak powers beyond current 10-PW-level systems by allowing the compression of much larger bandwidths potentially reaching the 100-PW-level. This detailed design and fabrication methods presented here would prove useful to advancing grating technology and benefiting the laser community. I recommend this manuscript for publication in Nature Communications.

Summary:
The authors reference key CPA techniques such as Ti:Sa CPA, BBO OPCPA, DKDP OPCPA, and wide-angle noncollinear OPCPA and some of their limitations. Reducing the pulse duration is identified as one of the more straight-forward ways to significantly boost peak power since current 10 PW lasers are nowhere near a single-cycle pulse duration. Current grating technology can support ~200 nm bandwidths in the near-infrared, but this new design will allow improved broadband diffraction efficiency for reaching single-cycle pulse durations. This article presents a design for 400 nm ultra-broadband gold gratings compatible with large azimuthal angles (ie. out-of-plane compressors) developed for near-single-cycle pulse stretching and compression. The authors identified free parameters that may be adjusted in the grating design and proposed a new design based on an optimized parameter space. In addition to optimization the tolerancing was also considered to ensure that the design was realistic to fabricate with common methods. The grating profile evolution was thoroughly studied to find the most reliable fabrication recipe. A 4 wt.% NaOH developer was found to broaden the base width while a 6 wt.% NaOH developer allowed the sharpening of ridges, both of which contributed to enhancing the efficiency over a broader bandwidth. The grating design was incorporated in a wide-angle NOPCPA system that could produce pulses as short as 6 fs, where the diffraction efficiency of the gratings could ideally compress a bandwidth sufficient for a 4 fs FTL pulse duration. The gratings exhibit a high diffraction efficiency > 90% for >400 nm bandwidth from 750 to 1150 nm, facilitating development of next-generation high-peakpower lasers. Comments: • Research is thorough and well-organized with appropriate figures showing the grating design compared to measurements of the fabricated gratings.
• The proposed grating design with optimized base width, groove depth, and shape factor is backed up with quality characterization and experimental implementation in an ultra-broadband OPCPA system.
• The fabrication and characterization methods are described in clear detail, which is very useful to others working to develop improved gratings for ultra-broadband lasers.
• The quality of the manuscript text and figures are high. half-meter-scale and tiled meter-scale gratings, as shown in Fig. R-1a.
The engineering project for full-aperture meter-scale grating is in progress. At present, our team has the capability to expose meter-scale gratings. Fig. R-1b shows a 1650 mm × 1120 mm off-axis mirror in the exposure system 5 . In addition, the meter-scale gratings will be completed in 2023-2024 in SIOM. 2. This work is in the proof-of-principle stage for the 400 nm ultra-broadband gratings.    Consequently, future work involves extending our results to meter-scale gratings for other groups or companies worldwide.

Question 2.
There is no evidence for uniformity of diffraction efficiency.

Answer 2:
Thanks for pointing this out.

Diffraction efficiency map of small size ultra-broadband gold grating.
In a scanning photometry map, the grating was measured over an area of 46 mm × 46 mm with a laser beam size of 2 mm, a step size of 2 mm. Each diffraction efficiency map contains 576 points, of which the number of single-point sampling is 5. 1443 and 1527 lines/mm ultrabroadband gold gratings were tested at 50° and 62° in TM polarization, respectively.   Figure S9 shows the effects of the weak non-uniformity and seam of the diffraction efficiency map (see Figure S8) on the 3D spatiotemporal structure of the compressed and focused pulse beam.
The following simulations can be divided into two main categories according to whether or not to consider the existence of seams in ultra-broadband gratings.    Nevertheless, the focused peak intensity is different in the above cases. Here, the focused peak intensity in Fig. S5 (b) (with perfect gratings) is normalized to 1. Accordingly, Fig. S10 shows  Since the wavefronts of G1 and G4 introduce only wavelength-independent distortion6,9, which can be easily eliminated by the deformable mirror, only the wavefronts of G2 and G3 in the compressor are considered in the following analysis.    Fig. S12 (b) and (c).   S13 illustrates the values of the normalized focus peak intensity in the above cases. Here, the focus peak intensity in Fig. S5 (with perfect grating) is normalized to 1. The normalized peak focus intensity decreases from 0.71 to 0.60 and from 0.66 to 0.28 as the modulation period decreases from 800 mm to 200 mm at a PV value of λ/3 and λ/2, respectively. For the degradation of the normalized focus peak intensity to be less than 50%, the modulation period should be larger than 200 mm, and the modulation PV should be less than λ/2, which can be satisfied by our current grating manufacturing capability. In addition, this kind of wavefront distortions can now be controlled or pre-compensated 9,10 , and the method is shown in Fig. S14 and introduced in Refs. [9,10].

Detailed damage testing experiments have been added to the main article.
The revised version is as follows:   . The pulse width, spectrum, and beam profile were measured in S1 or S2. b Measured and FTL pulse duration of the compressed laser pulse in S1. c Measured spectrum in S1 and S2. As shown in Fig. 9b, the compressed pulse width is 15±1 fs in S1. For the design of highpeak-power damage-test systems, self-phase modulation (SPM) must be considered. The spectrum ( Fig. 9c) in S1 and S2 shows a relatively weak self-phase modulation effect. The effective beam size is ~ 0.07 mm2 in the beam normal.
According to the ISO-21254 LIDT standard, 1-on-1 damage tests were performed in the air.
1443 lines/mm and 1527 lines/mm gratings were tested at 50° and 62° in TM polarization, respectively. Twelve sites were tested for each energy fluence. Note that the LIDTs were provided on the surface and determined by extrapolating the damage probability curve to zero probability." In conclusion, this is an interesting and encouraging result that offers a hint of the possible direction in broadband grating development for CPA, and this reviewer is the opinion that it will be of interest to the audience of a more specialized journal.

Answer:
We appreciate your positive comments as well as the questions you have raised. Those comments are all valuable and very helpful for revising our manuscript and improving our work. We are pleased to get your positive feedback and enthusiastic discussion!

Answers to Reviewer 2
Reviewer #1 (Remarks to the Author): The manuscript "400 nm ultra-broadband gratings for near-single-cycle 100 Petawatt lasers" by Yuxing Han  The authors reference key CPA techniques such as Ti:Sa CPA, BBO OPCPA, DKDP OPCPA, and wide-angle noncollinear OPCPA and some of their limitations. Reducing the pulse duration is identified as one of the more straight-forward ways to significantly boost peak power since current 10 PW lasers are nowhere near a single-cycle pulse duration. Current grating technology can support ~200 nm bandwidths in the near-infrared, but this new design will allow improved broadband diffraction efficiency for reaching single-cycle pulse durations. This article presents a design for 400 nm ultra-broadband gold gratings compatible with large azimuthal angles (ie. out-of-plane compressors) developed for near-single-cycle pulse stretching and compression. The authors identified free parameters that may be adjusted in the grating design and proposed a new design based on an optimized parameter space. In addition to optimization the tolerancing was also considered to ensure that the design was realistic to fabricate with common methods. The grating profile evolution was thoroughly studied to find the most reliable fabrication recipe. A 4 wt.% NaOH developer was found to broaden the base width while a 6 wt.% NaOH developer allowed the sharpening of ridges, both of which contributed to enhancing the efficiency over a broader bandwidth. The grating design was incorporated in a wide-angle NOPCPA system that could produce pulses as short as 6 fs, where the diffraction efficiency of the gratings could ideally compress a bandwidth sufficient for a 4 fs FTL pulse duration. The gratings exhibit a high diffraction efficiency > 90% for >400 nm bandwidth from 750 to 1150 nm, facilitating development of next-generation high-peak-power lasers. Comments: • Research is thorough and well-organized with appropriate figures showing the grating design compared to measurements of the fabricated gratings.
• The proposed grating design with optimized base width, groove depth, and shape factor is backed up with quality characterization and experimental implementation in an ultra-broadband OPCPA system.
• The fabrication and characterization methods are described in clear detail, which is very useful to others working to develop improved gratings for ultra-broadband lasers.
• The quality of the manuscript text and figures are high.
We are grateful for your effort reviewing our paper and giving an accurate summary of our work. We